Modified polymerization of vinyl aromatic compounds



Patented Mar. 13, 1945 MODIFIED POLYMERIZATION OF VINYL AROMATICCOIHPOUNDS Edgar c. Britton and Walter J. Le Fevre, Midland, Mich.,assignors to The Dow Chemical Company, Midland, Mich, a corporation ofMichie can No Drawing. Application April 18, 1942,

Serial No. 439,565

8 Claims. (Cl. 260-91) for the preparation of molded articles, thetougher resins having a molecular weight of say 70,000 or higher arepreferred, whereas for other purposes, e. g. as ingredients of lacquersor varnishes, vinyl aromatic resins of lower molecular weight are oftendesired. It is known that in preparing such resin the molecular weightof the product may be lowered by raising the temperature at which thepolymerization is carried out, by employing peroxides, acids, or othercatalysts to increase the rate of the polymerization reaction, or bycarrying the reaction out in the presence of a solvent for the reactingmaterial. The first two of these usual conditions for depressing themolecular weight of the product are often undesirable, since theyinvolve a considerable increase in the rate of the strongly exothermicpolymerization reaction and may result in the reaction occurringviolently and/or in the formation of a discolored product of non-uniformquality. Dilution .of the reaction mixture with a solvent usually causesboth a considerable reduction in the rate of reaction and a lowering ofthe averagemolecular weight of the product. In most instances theproportion of solvent required to reduce the molecular weight of theproduct, e. g. to one-halfof that of-the product obtained in the absenceof the solvent under otherwise similar conditions, is quite large'andadds considerably to the cost of the operation.

We have discovered that dicyclopentadiene influences the reaction forthe polymerization of vinyl aromatic compounds, or for theco-polymerization of such compounds with other unsaturated organiccompounds, in unusual manner. Its eiTect in causing a reduction in theaverage molecular weight of the polymericproduct is far greater thanthat of most organic solvents and is apparently a catalytic effectinstead of being the mere dilution effect of most solvents. The presenceof one per cent by weight or less of dicyclopentadiene during "thepolymerization of styrene is suflicient to cause the polystyrene productto be of markedly lower molecular weight than is polystyrene prepared inthe absence of dlcyclopentadiene under otherwise similar conditions. Theeii'ect of dicyclopentadiene in causing a reduction in the molecularweight 01' the product becomes more pronounced as the proportion thereofin the reaction mixture is increased from a trace, e. g. 0.5 per cent orless, to about 15 per cent by weight. Further increases in theproportion of dicyclopentadiene may cause a further lowering of themolecular weight of the polymer which is formed, but this effect becomesless pronounced as the proportion of dicyclopentadiene is increased, e.g. from 15 per cent to per cent of the weight'of the mixture; Thereduction in the rate of the polymerization reaction due to dilution ofthe vinyl aromatic com pound with dicyclopentadiene is usually nogreater, and in fact appears to be somewhat less, than occurs when thevinyl aromatic compound isv diluted with a like proportion of otherorganic solvents, e. g. benzene, toluene, or ethylbenzene, etc.

Examples of vinyl aromatic compounds which may be polymerized in thepresence of dicyclopentadiene to obtain polymers of lowered molecularweight are styrene, ortho-methyl-styrene, para-methyl-styrene,meta-ethyl-styrene, paraisopropyl-styrene, ortho-chloro-styrene,parachloro-styrene, divinylbenzene, vinyl-naphthalene, etc.Dicyclopentadiene, when present during the polymerization, is alsoeffective in causing a reduction in the average molecular weight oftheproducts obtained by co-polymerizing vinyl aromatic compounds withone another or with other unsaturated organic compounds such asmethyl-methacrylate, ethyl acrylate, divinyl maleate, or butadiene, etc.

Except for the requirement that dicyclopentadiene be present, thepolymerization or copolymerization of such compounds may be carried outin any of the usual ways, e. g. by polymerizingthe compounds in thepresence of the dicyclopentadiene only, or in the presence of otherorganic solvents or diluents such as benzene, ethylbenzene, etc., or inaqueous emulsion. Catalysts for promoting the rate of polymerization,such as hydrogen peroxide, benzoyl peroxide or mineral acids, etc., maybe present if desired, and when present may further depress themolecular weight of the polymeric product, but the presence of suchcatalyst is not required.

The dicyclopentadiene is efiective in causing a lowering of themolecular weight of the polymeric product regardless of the proportionin which it is used, and, as hereinbeiore mentioned.

the amount by which the molecular weight of the product is loweredincreases quite sharply as the proportion of dicyclopentadiene isincreased from a trace to about 15 per cent by weight. Thus, underotherwise uniform reaction conditions, the molecular weight of theproduct may be predetermined by the proportion of dicyclopentadieneadded. In practice we usually employ between 0.5 and 50 per cent andpreferably between 10 and 40 per cent of dicyclopentadiene, based on thecombined weight of the latter and the compound or compounds to bepolymerized, but the dicyclopentadiene may be used in larger proportionif desired. The dicyclopentadiene is usually added before efiecting thepolymerization, but there are instances in which a mixture of a polymerof high molecular weight and the corresponding polymer of relatively lowmolecular weight is desired and in such instance dicyclopentadiene mayadvantageously ing the polymerization reaction.

The polymeric product is often obtained in a form suitable for directemployment for the intended purpose, but when necessary it is furthertreated as usual to obtain the same in the desired form. For instance,when the polymerization has been carried out en masse or in solution,the dicyclopentadiene and any other organic liquids are usuallyvaporized from the product and the latter is crushed, out or otherwiseshaped into pieces of suitable size and shape. When the polymerizationhas been carried out in aqueous emulsion, the product is coagulated inany of the usual ways, e. g. by strong heating or cooling of theemulsion or by adding any of a variety of agents such as sodiumchloride, acids, etc., which are capable of causing coagulation, and theproduct is separated from the liquor, washed and dried.

The following examples describe certain ways in which the principle ofthe invention has been applied, but are not to be construed as limitingits scope.

Exmrts 1 In each of three experiments either styrene alone or a mixtureof styrene and dicyclopentadiene was heated at a temperature of 100 to110 C. for 66 hours to effect polymerization. The product was thenpurified by dissolving it in benzene, precipitating it from the solutionby admixing the latter with ethyl alcohol, separating the precipitatedpolymer and evaporating occluded liquor therefrom. The molecular weightof each polymeric product was measured by the well known Staudingerviscosity method. The following table gives the composition of materialsubjected to polymerization in each experiment and the molecular weightof the polymerized product.

be added durpentadiene in promoting the formation of polystyrene of lowmolecular weight with that of other organic liquids such as aresometimes used as mediums in carrying out such polymerization. Table IInames the ingredients of the material subjected to the polymerizationand gives the per cent by weight of each. It also gives the yield ofpolystyrene as per cent oi the weight of the styrene initially employedand the average molecular weight of the polystyrene product. In eachexperiment the polymerization was carried out by heating the reactionmixture at temperatures between 100 and 105 C. for 120 hours. Thepolymer was purified and the molecular weight was determined as inExample 1.

- with sufiicient nitric acid to give the emulsion a Table I Startingmaterials R N Di cl i??? U11 0. cy open- W6! %?5? "(310 px product I 3*weight mm Exmtu 2 pH value of 2. alone was that in one of theexperiments the The distinction between the emulstyrene-containingorganic liquid used in preparing the emulsion was styrene alone whereasin the other experiment it was a solution of styrene anddicyclopentadiene which contained per cent by weight of the latter. Theemulsions were each heated at a temperature of C. for -2 hours, afterwhich the resultant polystyrene product was precipitated by adding toeach emulsion 3 parts by volume of ethyl alcohol and boiling themixture. In each instance the precip'tated polystyrene was separated byfiltration, washed with alcohol, dried, and the molecular weight wasdetermined as in Example 1. Table III identifies the styrene-containingorganic liquid employed as a starting material in each experiment andgives the molecular weight of the polystyrene p Table IIIStyrene-containing organic ii uid q Molecular mm m r tdl i il c or can p$331 353- Other modes of applying the principle of the plained changebeing made as regards the method herein disclosed, provided the step orsteps stated by any the following claims or the equivalent or suchstated step or steps be employed.

We therefore particularly point out and distinctly claim as ourinvention:

1. The method which comprises polymerizing a vinyl aromatic compound inthe presence of dicyclopentadiene but in the absence or otherpolymerizable compounds, whereby a polymer is formed which is of lowermolecular weight than is obtainable by carrying out the polymerizationin the absence oi dicyclopentadiene under otherwise similar conditions;1 Y

2. The method which comprises polymerizing styrene in the presence ofdicyclopentadiene but in the absence of other polymerizable compounds.whereby a polymer is formed which is or lower molecular weight than isobtainable by polymerizing the styrene in the absence ofdicyolopentadiene under otherwise similar conditions.

3. The method which comprises polymerizing a vinyl aromatic compound inthe presence of dicyclopentadiene, but in the absence or otherPolymerizable compounds. the dicyclopentadiene being present in amountcorresponding to between 0.5 and 50 per cent of the combined weight ofthe vinyl aromatic compound and the dicyclopentadiene.

4. The method which comprises polymerizing a vinyl aromatic compound attemperatures between '10 and 170' C. in the presence oi.dicyclopentadiene, but in the absence of other polymerizable compounds,the dicyclopentadiene being present in amount corresponding to betweenand 50 per cent of the combined weight or the vinyl aromatic compoundand the dicyclopentadiene.

5. The method which comprises polymerizing styrene in the presence ofdicyclopentadiene, but

in the absence of other polymerizable compounds,

the dicyclopentadiene being present in amount corresponding to between0.5 and 50 per cent of the combined weight of the styrene and thedicyclopentadiene.

6. The method which comprises polymerizing styrene at temperaturesbetween 70 and 170 C. in the presence of dicyclopentadiene, but in theabsence of other polymerizable compounds, the

- pentadiene.

8. The method which comprises polymerizing styrene while in an aqueousemulsion thereof which also contains dicyclopentadiene.

EDGAR C. BRI'I'ION. WALTER J. LE FEVRE.

